We
are using scanning tunneling microscopy (STM) to explore the
electrical, structural and magnetic properties of a variety of
molecular species. We have developed an ultra-low current (down
to below
100
fA) UHV STM for studies of the electronic spin states of single
molecules.

Au
(111) surfaces

Graphite surfaces &
superlattices

Molecules on surfaces

STM
on Graphite

We are particularly interested in HOPG (Highly
oriented pyrolytic graphite), and in the many intriciacies of
performing STM on this material, which is one of the most commonly used
substrates for STM. Due to the weak coupling between adjacent
layers of graphene, graphite has extremely low sliding friction, hence
its use in lead pencils. This propensity for sliding also means
that the local stacking of sheets can be easily disrupted, and this can
be directly probed using STM. Through these sort of experiments,
we can learn alot about the intriguing properties of this material.

What happens when there is a mis-orientation of
graphene layers? We get a superlattice as shown below.
Here, we have shown two graphene sheets with atomic lattice constant,
d, mis-oriented by an angle Q,
which leads to the formation of an interference pattern (known as a
Moiré pattern - well-known from optics) with the same symmetry,
but a
superperiod, D. The relationship between the various quantities
is given by

D = d/(2sin(Q/2))

overlap
->

In an STM image
this looks like:

In this image, we can simulatneously observe the
superlattice as well as the atomic lattice. Note that the atomic
lattice has triangular symmetry, whereas the arrangement of the carbon
atoms is in fact hexagonal. This has been a subject of great
debate for over 25 years now, and is due to the AB (Bernal) stacking of
the individual graphene sheets. Essentially, the STM observes
every second carbon atom (known as b-atoms),
i.e.
the ones which are above a vacancy in the layer immediately below.

We have recently demosntrated that the STM tip can
be used to modify the spacing between the layers locally due to the
attractive forces between the tip and sample, and we can essentially
decouple the top layer, so that acts like graphene. This is
observed in the image below where we have modified the tip-sample
distance during a scan to decouple the top layer, whereby we see the
triangular lattice in the upper part of the image, and the true
hexagonal lattice in the lower part, whilst the top layer is
decoupled.

HOPG warps
under the
STM tip during scanning.
On the right image, for the bottom half, we have used the STM tip to
decouple (lift) the top layer of graphene, and in the top half, we have
reverted to the normal coupling.

This can be done in a controlled way as shown
below. Here we have a region on a piece of graphite (HOPG) where
there usual stacking of the graphene sheets has been disrupted,
resulting in a slight rotational misorientation between the top few
layers at the Basal plane. This manifests itself as a
superlattice , which is apparent on the right hand half of the
images. In the superlattice region, the average interlayer
spacing is slightly larger than usual, resulting in a reduced
electronic coupling. In the image on the left, the typical atomic
resolution image of HOPG is observed (displaying triangular symmetry,
thus showing every second C-atom) everywhere. In the image on the
right, where we have brought the STM tip closer to the surface, we
still observe the triangular lattice on the left half, but now obtain
the true atomic lattice (honeycomb, hexagonal symmetry) on the right
half. Due to the reduced interlayer coupling on the right, the
STM tip is able to lift the top layer there by a few tenths of a
nanometre, enough to decouple it almost completely, so it appears like
graphene.

No
decoupling: triangular lattice
partial decoupling: trianglar lattice on
left, honeycomb on right

Triangular lattice
Hexagonal lattice We can also
simulate these images using a simple model which we developed (see
publications list):

Triangular
lattice
Hexagonal lattice

We
have also gone a step further and have carried out a
study on the dependence of atomic mismatch between graphene sheets in
graphite and the misorientation of the resulting superlattice domains.
We have shown, through measurements of the atomic orientation of the
top sheet and the angular misorientation between superlattice domains,
that it is possible to compute the actual degree of atomic mismatch on
the underlying graphene sheet in the case of a superlattice. We show
that the odd-even transition is evident for superlattices with
relatively small periodicity in the range 1–2 nm and less apparent for
those with larger periodicity in the range 5–8 nm, and present a
signature of the transition. We also demonstrated that the degree of
interlayer coupling between graphene sheets depends on the extent of
rotational mismatch in relation to interlayer spacing as has previously
been predicted.

Defect-high region
of HOPG
showing
FFT from superlattice (measured) FFT
from superlattice (calculated)two superlattices
with different
period
showing satellite peaks
associated
showing identical features
stemming from two
graphene
sheets
with
superlattice & odd-even transitionwith different
rotational mismatch.Different period
also evident in FFT
(inset)